1. Field of the Invention
The present invention relates to a display device, and more particularly, to a backlight control unit and a liquid crystal display (LCD) device having the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for controlling an optical output of a backlight according to an image.
2. Description of the Background Art
Generally, an LCD device includes a liquid crystal panel with a plurality of pixels arranged in a matrix form, and a driving unit for driving the pixels. The liquid crystal panel includes a color filter substrate, a thin film transistor (TFT) array substrate attached to the color filter substrate, and a liquid crystal layer interposed therebetween. A common electrode and a pixel electrode are formed on the color filter substrate and the TFT array substrate, respectively, through which an electric field is applied across the liquid crystal layer.
FIG. 1 is a plan view showing red, green and blue pixels arranged on a liquid crystal panel in accordance with the related art. The red (R), green (G), and blue (B) pixels sequentially arranged on a substrate 110 in a matrix form, as shown in FIG. 1. Since the LCD device is not a light emitting device but rather an optical modulation device, a backlight is provided at a rear surface of the substrate 110. The amount of light emitted from the backlight transmitted through the R, G, and B pixels is controlled, to thereby display a desired image. More specifically, the R, G, and B pixels of the LCD device transmit R, G, and B wavelengths of light emitted from the backlight and absorb other wavelengths, respectively. Thus, the LCD device has brightness lower than a related art color cathode ray tube. Therefore, an LCD device having four sub pixels for implementing a white pixel (W) in addition to the R, G, and B pixels has been proposed.
FIG. 2 is a plan view showing red, green, blue, and white pixels arranged on a liquid crystal panel in accordance with the related art. The red (R), green (G), blue (B), and white (W) pixels are sequentially arranged on a substrate 210 in a matrix form, as shown in FIG. 2. As aforementioned, the R, G, and B pixels of the LCD device transmit R, G, and B wavelengths of light emitted from the backlight and absorb other wavelengths, respectively. The white pixel (W) increases brightness of a white color component that is also collectively generated by the R, G, and B pixels, thereby enhancing an entire brightness of the LCD device.
Each amount of light that passes through the R, G, and B pixels is determined according to an alignment direction of liquid crystal corresponding to the R, G, and B pixels. The alignment direction of liquid crystal is controlled by R, G, and B gradations constituting image data. The gradation is a value representing brightness of light that passes through each of the respective R, G and B pixels.
A white gradation implemented by the white pixel (W) is determined according to the R, G, and B gradations of the image data. For instance, the white gradation can be determined by applying a minimum value among the R, G, and B gradations or a squared value of the minimum value to the white pixel. However, there is a limitation as to how much the entire brightness of the LCD device can be enhanced by using white (W) pixels.
The limitation is due to the amount of space occupied by the white (W) pixel, which is a ¼ of the entire area among the R, G, and B pixels. Since the white pixel is very narrow, an overall light output amount of white light emitted through the white pixel is not very substantial. According to the conventional algorithm for determining a white gradation, if one or two of R, G, and B gradations is a minimum value, the minimum value is applied as a white gradation. Such an algorithm may result in the entire brightness of an LCD device having R, G, B, and W pixels being lower than that of an LCD device composed of R, G, and B pixels. Further, such an algorithm may result in only primary colors, such as red, green, blue, magenta, yellow, and cyan, being displayed if one or two of the R, G, and B gradations is at a very low value.